Mapping regional oxygenation and flow in pig hearts in vivo using near-infrared spectroscopic imaging

We used near-infrared spectroscopic (NIRS) imaging to assess the flow-dependence of both myocardial oxygenation and distribution of an intravascular tracer, indocyanine green (ICG). In open-chest pigs, nominal flow through the left anterior descending artery was reduced for 90 to 0 min (n = 6), 20 ± 1 (n = 5) and 44% ± 5% (n = 4) by variable occlusion and subsequently restored (to 219% ± 71% at 45 min) for 120 min. Electrocardiogram-gated NIRS images of the heart were obtained using a CCD-array camera with a liquid crystal tunable filter, which acquired absorbance spectra in the range of 650–1050 nm for each of 256 × 256 pixels (0.4 × 0.4 mm each). Deoxy- and oxy-(hemoglobin (Hb) + myoglobin (Mb)) levels were determined independently by applying a spectral fitting algorithm to the spectra between 650 and 890 nm. Maps for oxygenation parameter (OP = oxy-(Hb + Mb)/deoxy-(Hb + Mb)) and oxygen saturation parameter (OSP) (oxy-(Hb + Mb)/total-(Hb + Mb)) were constructed. To visualize flow distribution, a bolus of ICG (8.3 mg/5 ml) was injected I.V. at each step of the protocol and gated images were acquired at 800 nm every second over 60 s period. The ratio of ICG wash-in velocity to equilibrium absorbance (V/ΔAtail) was calculated. Changes in flow to 0%, 20%, 44%, 100% and 208% of baseline resulted in OP values of 1.46 ± 0.25, 1.66 ± 0.30, 2.22 ± 0.45, 2.78 ± 0.30 and 3.94 ± 0.33 in the affected area. Flow rates of 20%, 44%, 100% and 220% of baseline corresponded to V/ΔAtail values of 0.11 ± 0.11, 0.39 ± 0.13, 0.54 ± 0.17 and 0.61 ± 0.20, respectively. Thus, measurements of oxygenation and normalized ICG wash-in velocity correlated well with coronary flow, allowing for intraoperative optical assessment of the severity of regional ischemia.